JRE

JDK

To install the default JDK (Java Development Kit) on your system, run:

apt-get install default-jdk

Java and Debian

Debian provides several Java implementations. Each of them have a development environment (JDK) and a runtime (known as JRE, or Java Virtual Machines JVM). Here's a list of Java environment. Just run aptitude search "?provides(java-runtime)" on your system to list available packages).

The default JRE/JDK installed in Debian isn't the same for all architecture. Check the dependency of the package default-jdk (or run apt-cache depends default-jdk on a system).

Understanding Java Virtual packages names

There are several virtual packages used in Debian for Java. These cover runtime compatibility and come in two flavours; headless (omits graphical interfaces) and normal. A dependency on javaX-runtime means that the package can work with any runtime which implements version X. Runtimes should provide all virtual packages which they support, so openjdk 6 provides java2-runtime, java5-runtime and java6-runtime.

Switching

Most java-related executables, multiple of which can be installed at any time, are managed by the 'alternatives' system. Instead of managing each alternative separately, you can update them all in one go with 'update-java-alternatives' from java-common